It is known that a medical ultrasound imaging system can be used to display and analyze anatomical structures within a patient's body. The ultrasound imaging system transmits sound waves of very high frequency, typically 2 to 10 MHz, into the patient's body and processes echoes reflected from tissues and materials within the patient's body. A number of different types of displays are provided by ultrasound imaging systems but probably the most popular display is a two-dimensional image of selected cross-sections of the body. In an echo mode of operation, all echoes from a selected cross-section are processed and displayed. Use of the echo mode of operation enables a sonographer to detect a number of anatomical defects. Further, the size of such defects can be more or less precisely determined. The performance of the echo mode of operation is determined by the size of a resolution cell and, as is well known, the size of a resolution cell can be decreased by utilizing dynamic focusing and dynamic (matched) filtering.
In some clinical applications, anatomical defects can be relatively small, and echoes produced by such small anatomical defects are overshadowed by larger echoes from surrounding tissue. However, such small anatomical defects may be seen by displaying changes in blood flow velocity. As is well known, Doppler measurements can be used to determine the velocity of a moving object and a display of Doppler shifts caused by blood flow enables small anatomical defects to be detected more easily. This mode of operation wherein Doppler shifts caused by blood flow are displayed is known in the art as Color Flow. For example, U.S. Pat. No. 4,800,891 describes the Color Flow process and describes how Doppler information relating to blood flow velocity can be gathered from a large, selected cross-section of an anatomical structure. Modern Color Flow processors used in ultrasound imaging systems output estimates of three spectral moments of a flow signal, power, velocity, and variance and ultrasound imaging system displays typically provide information related to power or velocity.
It is difficult to acquire sufficient ultrasound data to develop an accurate, high resolution, blood flow image at a high rate. Thus, in order to obtain more precise Doppler information about blood flow velocity from a small cross-section area, as is well known, a spectral Doppler mode of operation is used. In the spectral Doppler mode of operation it is possible to devote more time to a selected small area. The results of the spectral Doppler mode of operation are conventionally displayed by means of a frequency spectrum and an audio signal.
Current ultrasound imaging systems providing a spectral Doppler mode of operation and a Color Flow mode of operation suffer from an inherent defect. Such current ultrasound imaging systems measure blood flow velocity in a blood vessel of interest by using a Doppler frequency shift which is obtained by analyzing echoes received from a region of interest from one receive beam direction. However, as is known, blood flow velocity measured in this way is a function of the angle of blood flow with respect to an ultrasound transmit beam. Thus, in the absence of information about the blood flow angle, the measured blood flow velocity is only a projection of the true blood flow velocity in the direction of the ultrasound transmit beam. In order to overcome this deficiency in the Spectral Doppler mode of operation, an operator, i.e., a sonographer, has to adjust the ultrasound transmit beam manually to align it with the direction of blood flow in the blood vessel to obtain a more accurate measurement of blood flow velocity. As one can readily appreciate, manual angle correction of blood flow velocity is only applicable to the Spectral Doppler mode, is cumbersome, and is hard to use to make repeated measurements having the same angle.
As is well known, to obtain the blood flow angle, one needs to receive echoes from a region of interest from more than one direction. Several proposals have been made in the past to solve this problem using multiple beam configurations. However, most of these proposed techniques require multiple transmit and multiple receive beams, all of which complicate transducer functionality and are, therefore, not practical for use in a clinical setting. These multiple beam configurations suffer from an additional problem in that they have to be adjusted to insonify the same region within a blood vessel.
Another technique is described in an article entitled "Angle Independent Ultrasonic Detection of Blood Flow" by G. E. Trahey, J. W. Allison, and O. T. von Ramm, IEEE Trans. Biomed. Eng., vol. BME-34, pp. 965-967, December 1987. This technique is based on tracking motion of a speckle pattern produced by blood to achieve flow direction information. The technique relies on a two-dimensional search of a Doppler image and is, therefore, computationally very intense. For that reason, the technique is not considered to be practical for real time Doppler modes of operation.
More recently a proposal has been made for another technique that comprises: (a) sonifying a sample volume with one transmit beam and (b) detecting two receive beams from two angles. This technique is disclosed in an article entitled "Vector Doppler: Accurate Measurement of Blood Velocity in Two Dimensions" by J. R. Overbeck, K. W. Brach, and D. E. Strandness, Ultrasound in Medicine and Biology, vol. 18, No. 1, pp. 19-31, 1992. In the disclosed technique, a first transducer is used to generate a transmit beam and a second and a third transducer, disposed on either side of the first transducer element, are used to detect receive beams at the same angle with respect to the transmit beam. The technique suffers in that it is limited to a specific transducer configuration and it utilizes a fast-fourier-transform-based mean frequency estimator which makes the disclosed method inaccurate or complicated.
Lastly, a proposal has been made by P. J. Phillips of Duke University in 1992 for still another technique that comprises: (a) sonifying a sample volume with one transmit beam and (b) receiving two receive beams from two angles. In this technique, the transducer aperture is divided into two sub-apertures. A transmit beam is generated at one sub-aperture and a receive beam is detected at the same sub-aperture. Next, a transducer beam is again generated at the same sub-aperture and a receive beam is detected at the other sub-aperture.
In light of the above, there is a need in the art for a method for determining blood flow angle in an ultrasound imaging system and for using this result to provide a quantitative color flow mode of operation.